The U.S. and Japan have called for an emergency meeting of the United Nations Security Council after North Korea claimed to have successfully tested a hydrogen bomb, a senior UN official told ABC News.

Before North Korea's announcement, a magnitude 5.1 earthquake was measured in the country near the site of an earlier nuclear weapons test, officials said Tuesday night. The trembler was detected about 12 miles ENE of Sungjibaegam and the South Korean weather agency said indications were that it was "artificial."

“We have perfectly succeeded in testing our first hydrogen bomb,” an anchor said on North Korean state TV. “It was one hundred percent capable from our own wisdom, technology, and power. We have now scientifically test-proved a miniaturized hydrogen bomb.”

"U.S. Forces Korea is aware of reports on North Korea's nuclear test today," a U.S. Pacific Command spokesman said. "We remain vigilant and are fully committed to working closely with our Republic of Korea ally to maintain security on the peninsula."

NSC spokesperson Ned Price said in a statement that the seismic activity took place in the vicinity of a known nuclear test site and that his agency has seen Pyongyang's claims of a nuclear test. However, he could not confirm North Korea's claims.

"We are monitoring and continuing to assess the situation in close coordination with our regional partners," Price said in a statement.

The site of Tuesday's quake is about 5 miles from the Punggye-ri nuclear site where a test was conducted in 2013.

A quake measured at the site at that time registered the same magnitude. It was later deemed to be a nuclear explosion.

According to Price, North Korea conducted its first nuclear test in 2006 and only twice since, not including today's unconfirmed test.

"We condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," Price said.

"We have consistently made clear that we will not accept [North Korea] as a nuclear state. We will continue to protect and defend our allies in the region, including the Republic of Korea, and will respond appropriately to any and all North Korean provocations," Price concluded his statement.

U.S. officials said that they will send up specially equipped "sniffer" planes to determine whether a nuclear test was conducted and, if so, what type of test was done.

One official said the U.S. doesn't believe that North Korea has the capability for a hydrogen bomb but can't be certain until testing is conducted.

Executive Secretary Lassina Zerbo of the CTBTO, an organization monitoring the world for signs of nuclear testing, said, "If confirmed as a nuclear test, this act constitutes a breach of the universally accepted norm against nuclear testing; a norm that has been respected by 183 countries since 1996."

In its statement, the CTBTO urges North Korea to "refrain from further nuclear testing and to join the 183 States Signatories who have signed the Comprehensive Nuclear-Test-Ban Treaty," which bans all nuclear explosions.